The present invention generally relates to a flat-panel display device which comprises memory modules added to display pixels and used in a still image display mode, and more particularly to a flat-panel display device in which the display pixels are electrically separated from the memory modules in an ordinary display mode other than the still image display mode.
For example, liquid crystal display devices are widely used as monitor displays for portable information terminals such as portable phones and PDAs (Portable Digital Assistants), since the devices have such characteristics as thinness, compactness and lightness. The portable information terminals generally operate using power from a rechargeable battery. Thus, the available period of the portable information terminal considerably depends on the rate of consuming the battery power. Under the circumstance, active research has been made to reduce the power consumption of the liquid crystal display device.
In recent years, the memory technology represented by SRAM (Static Random Access Memory) is used for reducing the power consumption of the liquid crystal display. With SRAM technology, memory modules are added to the display pixels forming a display screen. Each memory module is electrically connected to a corresponding display pixel by a connection controller. When an external driving circuit supplies a video signal in this state, the video signal is captured by a pixel switch and supplied to the display pixel. The memory module holds the video signal supplied to the display pixel and drives the display pixel according to the video signal. Thus, in the case where frequent update of the display signal is not required, a still image can be displayed by causing the output operation of the external driving circuit to be intermittent.
In the field of liquid crystal display devices, frame-inversion driving is generally known. In this driving, the polarities of video signal voltages to be applied to the display pixels are inverted, for example, every vertical scanning (frame) period in order to prevent uneven distribution of liquid crystal materials. In addition to frame-inversion driving, H-line inversion driving and V-line inversion driving are known and used for suppressing generation of flicker. In H-line inversion driving, the polarities of the voltages are inverted in units of display pixels of one or more rows. In V-line inversion driving, the polarities of the voltages are inverted in units of display pixels of one or more columns. In liquid crystal display devices containing memory modules, the H-line inversion driving is used in the ordinary display mode and the frame-inversion driving is used in the still image display mode, for example, to further reduce the power consumption. The connection controller is used not only for controlling the electrical connection between the display pixel and the memory module, but also for controlling the polarity inversion described above.
However, with respect to the liquid crystal display devices containing memory modules, it is reported that a significant number of defective pixels appear on the display screen in the ordinary display mode.